The sailing art is replete with both conventional and unconventional sail designs. Certain of these devices are shaped like the wing of an airplane. For example, "Curious Yachting Inventions" by Joachim Schult (ISBN 0-808-2104-1) discloses several wing-shaped sail designs. FIGS. 56 and 57 show a sail which is inflatable in order to give it an airfoil shape. FIG. 73 depicts the Dyna-Ship which has rigid airfoils instead of cloth sails, and these are operated by remote control from the bridge. They are set on hollow, one-piece masts of variable elliptical sections. The airfoils are roughly trapezoidal in shape, and similar to the paddles on a turbine wheel, are set at decreasing angles to the wind looking forward. FIG. 74 describes a trimaran whose five vertical airfoils can be folded down when the boat is in harbour. The 28 ft long prototype goes as close as 6.degree. to the apparent wind, as opposed to 20.degree. for boats with cloth sails. In spite of its sail area of 323 sq.ft, which is large in relation to the hull weight, the vessel cannot capsize, because the wind flow always meets the sails at the optimum angle. The whole set of airfoils is always angled to the wind in such a way that they produce the maximum drive with the minimum resistance. FIG. 75 comprises an adjustable profiled airfoil to which a cloth sail is attached. can determine the most favourable profile which would give the least resistance with the maximum or drive. FIG. 77 consists of an improved airfoil design which allows the curvature of the sail to be selectively changed. FIG. 83 shows a propulsion system in which several airfoils rotating around a common axis are mounted on a revolving disc. FIGS. 86 and 87 depict a pivoting airfoil which also moves fore and aft and athwartships. The design reduces flow-pressure on the rotation axis and facilitates the trimming of the airfoil sail. FIG. 88 includes a multi-airfoil sail in which it is possible, with the help of parallel struts, to move the two outer airfoils forward or backward in relation to the central one without noticeably changing the angle of incidence. FIG. 89 consists of airfoils which freely pivot around a vertical axis. A vane is set to port or starboard and thus creates negative pressure on the convex side of the sail, which sets itself at an angle to the wind and consequently produces drive. "Windship Technology--Proceedings of the International Symposium on Windship Technology (Windtech '85)", Southampton, U.K., Apr. 24-25, 1985 edited by C. J.Satchwell, ISBN 0444425330 (set), LCCN 85016170//r88, discloses numerous wing sails, mostly of rigid construction for larger ships.